Outline of the activity:
• Onion cells are examined microscopically.
• DNA is extracted from onions using washing-up liquid.
• Children gain experience in designing and carrying out
a simple experiment.
5. Exploring
DNA:
Looking at
onion DNA
25 minutes
Intended message:
• That DNA is found inside cells (reinforcing activity 3).
Step 1
Use worksheet 9 to trace a path from the whole onion to the single cell, to inside the nucleus of that cell. This reinforces the idea that living
things are made up of lots of cells and that the ‘Recipe of Life’ is inside the nucleus of each cell.
Step 2
Cut an onion into large chunks and show that it is possible to peel off a very thin skin between the onion layers. This skin is one to two
cells thick and, if laid flat on a glass slide, it is easy to see the cells under the microscope. (Children can do this themselves if you don’t
mind them smelling of onions!)
Sue’s advice: To prepare the onion cell slide, cover a small piece of onion skin with a cover slip and seal the edges with
clear nail varnish. Examine using x10 objective and x10-x40 lens or equivalent, with illumination from underneath. The cells
will be clearly visible, and the nucleus should be obvious in some. If you can get hold of red onions, the features are clearer.
It is also possible to see cells by holding up the onion skin to the light and looking through a x10 hand lens although it can take the children some time to master this.
You will need:
• Worksheet 9 either as
individual copies, an OHT or
as an enlarged poster.
• Microscopes (3-4 if possible).
• Onion skin mounted on
microscope slides.
• Worksheet 10.
• The materials listed on
worksheet 10.
Step 3
Using the sandwich box cell and the balloon nucleus, remind the children of the discussion at the end of
Activity 3 concerning how to get the DNA out of the cells by breaking them open and bursting the nuclei.
Sue’s advice: children usually suggest breaking the cells open with a pin or knife. This can open
up a discussion of the problems associated with the small size of the cells and the idea that the
blender/liquidiser would be a good way of cutting cells open. Removing the nuclear (and cytoplasmic)
membranes requires washing-up liquid to break down the fat. The children should be told this fact but
you can then lead them into a discussion about the properties of washing-up liquid for removing fat
from dirty plates and relate this to the way in which washing-up liquid will act on the fatty parts
of the cells. ➔
Step 4
Use worksheet 10 to complete the DNA from onions activity. The basic steps and Sue’s advice are given below.
• Blend together the onions and the DNA extraction solution.
5. Exploring
DNA:
Looking at
onion DNA
(continued)
Sue’s advice: although I made all of the workshop solutions, there is clearly scope for the children to play a more active role if
time allows. Accurate “scientific” equipment is not needed: 100g of onion material can be weighed out on normal kitchen
scales, 3g of salt is about half a teaspoon and 10ml of washing-up liquid is two teaspoons. The washing-up liquid must be of the
“non-concentrated” type (i.e. cheap). If you are making the extraction solution in advance, it should be stored in the fridge it will be OK for 2 - 3 days).
• Point out that the liquidised mixture still contains quite large bits of onion - but that you must not liquidise the material any longer than 5
seconds for fear of breaking up the DNA. Discuss how to get rid of the large lumps.
Sue’s advice: the children always came up with the answer of sieving/filtering without much prompting. This is ‘least messy’
using a filter from a coffee machine but a tea-strainer or small plastic sieve would also do the trick.
• Explain that DNA is in the filtered liquid but that it is invisible at the moment because it is dissolved. (A useful analogy is that of sugar
dissolving in hot tea). In order to see the DNA, we need to make it “undissolve”. This is done by adding methylated spirits since DNA
does not dissolve in alcohol. This part of the activity will work better if the meths is cold, but this is not essential.
Sue’s advice: I explored the idea of ‘alcohol’ by giving familiar
examples like whisky, gin and surgical spirits (children who had had
their ears pierced often recognised this name). Many children had
actually heard of methylated spirits. ➔
W ashing U p
L iquid
• The filtered liquid and the meths are mixed in equal proportions; this can be
demonstrated by you on a large scale or a small sample can be given to each
child in a small plastic vial - which is more fun.
The mixture is shaken to precipitate the DNA.
If the children shake their own mixture, make sure that the lid of their vial is
fixed securely in place before they shake it and that you collect in the samples
after the activity.
5. Exploring
DNA:
Looking at
onion DNA
(continued)
Sue’s advice: I dispensed the liquids into small plastic vials and then let the
children shake to mix. From a safety point of view, it is best to keep children
right away from neat meths, particularly the purple version which is quite
noxious. The vigour with which the mixture can be safely shaken seems to vary
regionally! In North Wales, even violent shaking still produces beautiful results.
I suggest that you test this out using a small portion of the mixture, before
allowing the children to shake their mixtures.
•A useful discussion point is the length of the DNA. Point out that
DNA is a very long thin chemical (this builds on the information
the children have gained from activity 4).
Ask the question: “How far would the DNA from one cell stretch?”
The answer is approximately 1.5 metres. See if the children
remember (from activity 3) that our bodies contain over a million
million cells. This means that the DNA from a human body would
stretch for over a million million metres (to the moon and back
over 800 times). This fact never fails to impress the children, but
it is also useful for pointing out that the DNA has to be very long
to fit in all of the “recipes” needed to make a very complicated
human being.
Extension activity
Children might like to write their own set of instructions about
how to extract DNA from onions•